feat(frontend): hide mode toggle button until baseline is fully tested

Backend bug fixes (duplicate message, tool_calls compression, stale
thinking blocks, stop_reason) are ready to ship. The toggle UI is
hidden with the HTML hidden attribute — remove it to re-enable.

autogpt_platform/frontend/src/app/(platform)/copilot/components/ChatInput/ChatInput.tsx

@@ -160,8 +160,11 @@ export function ChatInput({
               onFilesSelected={handleFilesSelected}
               disabled={isBusy}
             />
+            {/* Mode toggle hidden until baseline transcript support is fully tested.
+                Backend logic is ready — this just hides the UI button. */}
             <button
               type="button"
+              hidden
               disabled={isStreaming}
               onClick={() =>
                 setCopilotMode(